The invention relates to a cold pack utilizing the negative heat of solution produced upon the dissolution of a material in a liquid.
Compact, self-cooling devices that produce cold through the negative heat of dissolution of a material in a liquid are known to the art. U.S. Pat. No. 3,804,077 to Williams discloses a cold pack which contains a water-soluble material (ammonium nitrate) and a starch material acting as a gelling agent in one zone, and water in another zone. When a user breaks a separator between the zones, the ingredients are mixed. Dissolution of ammonium nitrate is endothermic, absorbing heat from the surroundings and "producing cold." Starch gels are known to provide some rigidity to cold packs, inhibiting uneven distribution of the mixed contents of the pack under the influence of gravity. The effective cold-providing lifetime of such a cold pack is lengthened, because water is inhibited from reaching the ammonium nitrate as rapidly as would occur in the absence of a gel.
The starch tends to accumulate on the bottom of the pack during shipping, since the materials in the pack have very different particle sizes and densities. The ingredients of such packs are often poorly distributed. Some regions of the pack are overly well gelled, but they are not cold because the gel hinders diffusion of water and ammonium nitrate toward each other. Other regions of the pack may be inadequately gelled, and they are overly cold with a shortened cold-producing lifetime. Before use, the packs should be agitated to attempt to distribute the starch evenly. Uniform distribution is not always possible. This leads to inconsistent pack-to-pack gelling performance and consequently, temperature drop.
Additionally, powdered starch is used in many prior art devices. Powdered starch is used in order to provide rapid gel forming ability in cold solutions. It is very light weight and tends to produce dust during cold-pack manufacture. Starch dust contaminates the seal areas in plastic bag packages, so that imperfect or weak seals are formed. A significant portion of the powder is lost in dust control systems, leading to increased cost for the individual cold packs. Additionally, powdered starch is an extreme explosion hazard, creating unsafe working conditions in factories manufacturing cold packs.